1. Field of the Invention
This invention pertains to a method for coating particles.
2. Description of Related Art
Particles can be coated using a variety of techniques including fluidized bed, sol-gel, sputtering and evaporation. Since all of these techniques are batch type, meaning that the coating processes are not continuous and must, therefore, be filled and emptied, high product yields necessitate large processing chambers or a multitude of smaller chambers. Spray drying has been used to produce powders from precursor solutions requiring chemical reactions or from slurry mixes of the final components. Marsh U.S. Pat. No. 4,713,233 used spray drying to produce porous oxide powders from alkoxide solutions. To-date, this process has not been used for coating particles.
Although coated particles are used in many different applications, an important one is in electronic display devices which are visual interfaces between users and the information the users seek.
Some commercial applications of flat panel displays include laptop computers, avionic displays, automobile dashboards, navigation displays, video phones, medical systems, pocket notepads, and miniature displays. Military applications include wall-size command control displays, avionic displays, navigational displays, and head-mounted displays for soldiers.
The desired flat panel displays are thin, lightweight, have good brightness, have sharp contrast, colors, and have wide viewing angle. For example, displays used by soldiers on the battlefield should be able to withstand harsh environmental conditions, should be lightweight, should provide a wide viewing angle, should provide viewing in bright light, and have high resolution.
Various display technologies exist or are under development for flat panel displays. These flat panel displays include active matrix liquid crystal displays, electro luminescent displays, plasma displays, and field emission displays (FEDs). Each technology has its own merits and demerits and finds applications in various niche areas.
Flat panel field emission displays contain millions of micro-sized field emitters-arranged in a matrix. These field emitters are addressed, in a matrix, a pixel row at a time. The emitted electrons are accelerated toward the pixels on a screen a few millimeters away by an accelerating voltage. Each pixel is addressed by a large number of field emitters. An individual pixel consists of red, green and blue sub-pixels. Based on the desired color from an individual pixel, the corresponding sub-pixel is addressed and the phosphor is excited producing its characteristic color.
Phosphor particle selection and their requirements vary based on the conditions of their use. Field emission displays work under relatively low voltage of about 50 V-10 kV and relatively high current density of about 1-100 μA/cm2 in contrast to cathode ray tubes that operate at high voltage of about 15,000-30,000 volts and low current density. At the low accelerating voltages used in field emission displays, slow impinging electrons do not penetrate very deeply into bulk of the phosphor, further increasing the current density at the phosphor surface. If the phosphor surface is resistive, this high current density can lead to serious charging, local heating, and thermal breakdown.
Most of the commonly used phosphors in cathode ray tubes, field emission displays and electroluminescent displays are sulfides that have highly resistive surfaces and typical particle sizes in the range of 1-10 microns. Some commonly used cathode ray tube phosphors are ZnS:Ag, Cl (blue), ZnS:Cu (green),Y2O3:Eu (red), and Y2O3S:Eu (red) which are now being modified for field emission display applications. Some of the newly developed electro luminescent display phosphors, that maybe used in field emission display devices, are also sulfides:CaGa2S4:Ce (blue), SrGa2S4:Ce (blue), Zns:Tb (green) SrGa2S4:Eu (green), CaS:Eu (red), (Ca, Sr)Ga2S4:Ce, (Ca, Sr)Ga2S4 and mixtures thereof. Under high coulomb charging, the surface temperature of resistive phosphors increases thereby resulting in dissociation and surface degradation or aging. Sulfur dioxide and hydrogen sulfide gases evolve from the phosphor surface and can damage the field emitters. The longer address times associated with the high current densities used in field emission displays make the conditions worse and result in severe current saturation and phosphor degradation. In many cases, aging is accelerated by heat that is associated with phosphor charging. Since efficiency of the phosphor decreases as the accelerating voltage is decreased, the current density is increased to maintain brightness. The efficiency is further lowered by the high current densities involved in the process due to surface charging and aging. Hence, the phosphors currently used in field emission displays have very poor efficiency and service lifetimes.
Improvements in the efficiency of existing phosphors can be realized by reducing their resistivity, controlling their grain size, and modifying the surface chemistry of the phosphor particles. The efficiency of a phosphor can also be increased by using the quantum confinement effect. This is achieved by using quantum dots, i.e., less than 10 nm-sized particles. However, the nanocrystalline quantum dot phosphors have very large surface areas that result in electron traps from impurities at the surface and the traps reduce the observed efficiency of the nanophosphors. Each individual nanocrystalline quantum dot must be isolated or prevented from agglomeration to observe quantum effect. All the problems, such as surface degradation, environmental effects, and aging, discussed in the case of microcrystalline or large phosphors, still exist in the case of nanocrystalline phosphors. These problems can be solved by using appropriate protective coatings on the phosphors used in electroluminescent displays, cathodoluminscent displays, and field emission displays.
It should be understood that in addition to phosphor particles, other organic, inorganic and inorganic/organic hybrid particles can be used in the coating method claimed herein for other applications.
Electrically conducting, non-conducting, luminescent and other coatings can be used to isolate the particle from its surroundings and thereby protect the particle from degradation or reaction with its environment. The coating can also be used to protect the environment from species evolving from the particle. The primary use of this type of coating is to protect phosphors in field emission display devices. Phosphors degrade as a result of electron bombardment and the resulting electrical charging and heating. The coating serves to encapsulate the phosphor and, if it is a conductive coating, to conduct electrical charge away from the particle surface. If the coating is luminescent, one or more of such coatings can be placed on a particle to change the light spectrum of the system. This can be used in emissive displays and in solid state lighting devices.
In structural applications, the coating can be used to change interfacial thermal expansion coefficients, thermal conductivity, deposit mechanically advantageous grain boundary materials, such as grain pinning, grain size, shape and distribution, and provide a lower temperature sintering aid. The coating can also be used to shorten diffusion paths in solid state synthesis procedures, sol-gel and spray pyrolysis coating methods. In the sol-gel coating technique, the particles are coated while in suspension in the precursor solution. Once the particles are coated, the solvent is removed. This results in a batch type process that does not readily lend itself to industrial scale-up. Sol-gel coating methods also suffer from the need to exercise considerable care in maintaining exact pH and temperature conditions to prevent precipitation of sol-gel phases. Furthermore, the sol-gel phase may not nucleate on the powder particles thereby producing poor coatings and individual particles of the gel phases. Multi-component sol-gel phases may precipitate out of solution as individual phases. Spray pyrolysis method is used to manufacture particles from solutions containing dissolved species. The solution is sprayed into a hot zone or drying chamber. The solvent evaporates and leaves behind particles of the desired composition.
For present field emission display commercial applications, phosphors operating under accelerating voltages of 50-10,000 volts, must last in excess of 10,000 hours of continuous operation without losing 50% of the original brightness. Presently, the phosphors do not meet this standard. In an unobvious and an unexpected aspect of this invention, the invention disclosed herein makes it possible for such phosphors to meet the standard.